Abstract: According to the fact that the measuring pressure in the test mines by cross rock drilling is much lower than its adjacent mine, a numerical simulation is applied to analyze surrounding rock plastic zone in test site by Comsol Multiphysic software. The results show that when the shortest distance of the roadway is less than 7 m from the seam, the surrounding rock loose circle has been passed through the seam. In the case，it can not meet the requirements by use of conventional cement mortar sealing under pressure, and the main reason caused the lower pressure test results is decompression of the cracks passing through surrounding rock loose circle and the pressure chamber. So we should choose the pressure sealing method of "two ends blocking and middle injection", and the injection pressure must be more than 6 MPa.